1. Field of the Invention
The present invention relates to a microcomputer capable of reducing the quantity of hardware components, in which system there are provided a central processing unit performing various arithmetic operations as well as logical operations and other data processing tasks and auxiliary devices for performing various kinds of auxiliary functions. Particularly, the present invention is most advantageously applicable to the microcomputer which has components for the central processing unit and the auxiliary functions on a single semiconductor chip integratedly.
2. Description of the Related Art
In order to realize a desired computer system, a processor which executes the data processing in accordance with a user program stored in a main memory has to be combined with some of auxiliary devices which are purposively selected. As the auxiliary device in a sense here, various kinds of peripheral devices are of course included; for example, a direct memory access (DMA) controller for controlling a high-speed transfer of a large amount of data within the main memory, a refresh controller for controlling the refresh of dynamic random access memories, a timer, various kinds of interfaces and so on. Further, as is understood from the explanation described later, even a part of functions of the central processing unit can be of the auxiliary function in a certain case.
In case such a computer system is constructed by elements of a so called discrete type in which a circuit for the particular element is formed on a separate semiconductor chip, the complicated wiring of signal lines connecting the elements is necessiated and the control between the mutually related elements becomes also complicated.
Therefore, as is well known, a single chip microcomputer is proposed which has the central processing unit and some of the auxiliary devices in accordance with the necessity on the single semiconductor chip to achieve a particular function. For example, the microcomputer which has the central processing unit and the DMA controller as one of the auxiliary devices on the single semiconductor chip is taken into consideration as a conventional type of the single chip microcomputer.
According to the arrangement usually adopted in such a single chip microcomputer, two groups of devices with the structure similar to each other are formed on the single semiconductor chip, each having an arithmetic logic unit, registers, a buffer register and an internal bus line connecting them. One group functions as the central processing unit and the other as the DMA controller. The arithmetic logic unit generally has the functions of various kinds of arithmetic operations and logical operations. The arithmetic logic unit of the central processing unit uses both functions to execute the predetermined processing in accordance with instructions and data stored in the main memory, and that of the DMA controller uses the function of the arithmetic operation to conduct the operation of address referred to in the main memory and the calculation of the number of words to be transferred. There are a few cases where the function of the logical operation in the arithmetic logic unit of the DMA controller is utilized.
The registers provided in the processing unit and the DMA controller are used only for the operation of the respective arithmetic logic units, that is to say, the arithmetic logic unit of the processing unit reads out the data necessary for its operation from the registers belonging to the processing unit, and the result of the operation thereof is stored into those registers. The same is true of the relationship between the arithmetic logic unit and the registers in the DMA controller. The registers of the processing unit is referred to as a general-purpose register and those of the DMA controller as a peripheral register, hereinafter. The structure and function, however, are the same in both the registers, as described above.
Further, the buffer registers included both in the processing unit and in the DMA controller are also the same in their function and role. Namely, the buffer register is connected to an external bus line, to which the main memory is connected, for example, as well as the internal bus line. The data sent through the internal bus line are temporarily stored in the buffer register and outputted to the external bus line therefrom, and further the data transferred through the external bus line are temporarily stored in the buffer register and then taken into some other components in the processing unit through the internal bus lines.
As is described above, in case the single chip microcomputer is constructed with the processing unit and the DMA controller on the single semiconductor chip independently, microcomputer, in result, has the components of the same function in duplication, so that the quantity of the hardware components becomes larger and the occurance rate of logic failure becomes high because of the increase in the logic scale.